Path and line selection for protection in a network, such as, for example, a Synchronous Optical Networking (SONET)/Synchronous Digital Hierarchy (SDH) network, has historically been accomplished via selection in a central switch fabric or data ingress or egress of a port card.
Historically, switch fabrics have been built with specific technology for the signals it is expected to switch. Depending on the approach, many Time-Division Multiplexing (TDM) switch fabrics employ protection selection at an ingress, central switch fabric, or egress, which involves building bandwidth-scaled switch fabrics.
Ingress selection typically relies on a fixed physical relationship between line cards and the exchanging of TDM signals external to the central fabric. Egress protection selection provide flexible protection associations; however, they require building fabrics with extra capacity to support switching of at least two paths to an egress line card. Central fabric selection is a versatile approach; however, for fast switch-over times, the fabric must be built with protocol awareness and the ability to detect and react to protection switches quickly.
For generic packet-based switches carrying TDM signals, the central switch fabric is unaware of the services that it carries and is unable to make any decisions as to the selection of a protected data path.